204 Transactions of the Academy of Science of St. Louts 
smaller pebbles to bring the total number to 400. In some in- 
stances a far larger number of pebbles was analyzed, from a 
single exposure, and in one instance, where only a small quan- 
tity of drift was secured, the number of pebbles was less than 
400. Every effort was made to avoid personal selection. These 
pebbles were cracked and the fresh surfaces examined under 
the binocular microscope. Dilute hydrochloric acid was used to 
identify limestone, and hot hydrochloric acid to identify the 
finely broken dolomite. The results of several pebble counts, 
using this method, are shown in Table III. 
Since some of the samples of Illinoian drift came from ex- 
posures from which the calcareous material had been leached 
and other samples did not, it seemed desirable to disregard all 
calacareous and dolomitic pebbles in calculating the percent of 
northern pebbles present in the sample. This gave a result 
which might be described as the per cent of northern pebbles 
among the total insoluble pebbles, the word insoluble referring 
to those insoluble in hydrochloric acid. This percentage was ob- 
tained by dividing the sum of the igneous and metamorphic 
pebbles by the total number of pebbles after the limestone and 
dolomite pebbles had been subtracted. The results of these 
analyses may be epitomized as follows: 
Per cent of foreign pebbles among the insoluble pebbles 
Maximum Minimum Average 
Pre-Illinoian 13.6% 19% 5.9% 
Illinoian 36.8% 17.1% 24.6% 
Wisconsin (one only)......49.6% 49.6% 49.6% 
From this data it appears possible to differentiate pre- 
Illinoian drift from Illinoian drift in this locality on the basis 
of the percent of northern pebbles among the insoluble ones. 
Pre-Illinioan drift carries less than 15 per cent foreign pebbles. 
among the insolxble ones; Illinoian drift, 15 per cent or more. 
But comparison of these analyses with those MacClirtock made 
